The politically active rockers are headlining the L.A. Rising festival. And yes, they still have some radical messages to deliver.

In a rehearsal room in North Hollywood are some of the battle scars Rage Against the Machine has accumulated during many years of conflict and noise. In one corner are the scorched, graffiti-covered amplifiers that bassist Tim Commerford has plugged into for nearly 20 years. At Woodstock 1999, he had the big cabinets draped with a U.S. flag, which he then soaked with lighter fluid between songs, until the final encore of "Killing in the Name."

Thousands of fans were already bouncing and shouting along to the angry, defiant chorus ("I won't do what you tell me!") when Commerford pulled out his lighter. "I lit it up, and I swear to God the flames were 20 feet in the air," he says excitedly, standing tall in olive-drab shorts and a snug T-shirt. "It was awesome — honestly, one of the most high-profile flag burnings since the Vietnam War."

Commerford, 43, is here with Brad Wilk, 42, whose kick-drum displays the band's vivid red star of revolution. And soon guitarist Tom Morello arrives, stepping into the room in a blue baseball cap for his beloved Cubs, looking up from his BlackBerry. "The show's off!" says Morello, 47, with a laugh.

The show is a big one: the L.A. Rising festival Saturday at the Los Angeles Memorial Coliseum, where Rage Against the Machine will headline a day of music and politics, with performances by Muse, Rise Against, Ms. Lauryn Hill, Immortal Technique and El Gran Silencio. Also on-site will be the festival's "Re-Education Camp," with booths for activist groups on war, poverty, labor and immigration.

In a few hours, the musicians will be joined by singer Zack de la Rocha, 41, for a full band rehearsal, as they edge closer to the festival and the band's 20th year since first jamming together in the obscurity of Sunbird Studios not far away. They are in good spirits.

"We'll be fine," says Morello casually of the practice schedule. "The first day always sounds a little bit shaky. The second day is fine. The third day it sounds like Rage Against the Machine. Fourth day it sounds exactly how it's supposed to. The fifth day sounds worse than the fourth day." He laughs. "You don't want to do seven days. You want to save it."

The band discovered its sound and mission quickly at the beginning of the '90s, mixing punk, hip-hop and hard-rock into a swirling, muscular whole. Morello's metallic guitar erupted with wild, percussive force, mimicking staccato turntable effects and channeling Jimi Hendrix with agitated riffs and blistering funk. The vocals were pure hip-hop and delivered with such furious drive by De La Rocha that they worked as a livid hard-rock roar, equal parts Black Flag and Public Enemy.

Just as crucial to the band's identity was the political content, sending radical messages on social justice to the kids via MTV and commercial radio in the form of such signature songs as "Bulls on Parade" and "Testify." (Sample lyric: "Weapons not food, not homes, not shoes / Not need, just feed the war cannibal animal.")

From stages around the planet, Rage railed against war, torture and U.S. foreign policy, and stood for abortion rights and the struggles of immigrants, farmworkers and sweatshop laborers. Riot police were often on standby outside Rage concerts, and the rockers have been regularly tear-gassed along the way. "It will definitely kick your body and your mind into overdrive when that … is going down," Wilk says.

That sound suddenly went silent in 2000, in what appeared to be an especially acrimonious and permanent breakup, just as the Bush years were set to begin. "We weren't communicating, and we were losing the friendship," says Commerford, who first met De La Rocha in grade school in Irvine. But in one of their final acts before splitting apart, the band erupted for 45 staggering minutes onstage in the "free speech" zone outside the Democratic National Convention in Los Angeles.

"Brothers and sisters, our democracy has been hijacked," De La Rocha announced to the crowd of several thousand protesters and fans, denouncing the American two-party system and what he called its collusion in political and cultural repression. Afterward, riot police descended amid a storm of rubber bullets and batons. It was a career-defining scene.

"It was one of those moments where the hair on your arms was standing up. It was really something," remembers Paul Tollett, president of Goldenvoice Productions, Rage's partner for the Coliseum show. "Rage is always there. They are a voice for people who are fed up."